64-bit iOS support (one more try)

Added a new build configuration “Release 64bit” that overrides the
Architecture setting in the iOS targets to add 64-bit flavors.
To use it, edit the scheme (CBL iOS or Listener iOS), select Archive in
the list at the left, and change the “Build configuration” pop-up to
“Release 64bit”. Then dismiss the dialog box and choose Product > Archive.

I’ve tried not to disturb the existing configurations or targets, so
this shouldn’t break anything on Xcode 4.6 (famous last words!)